Aluminum anodizing apparatus



United States Patent 3,473,103 ALUMINUM ANODIZIN G APPARATUS Fred C. Schaedel, Los Angeles, Calif., assignor to Murdock, Inc., Compton, Calif., a corporation of California Original application Feb. 28, 1966, Ser. No. 530,508, now Patent No. 3,418,222, dated Dec. 24, 1968. Divided and this application Aug. 30, 1968, Ser. No. 756,586 Int. Cl. H02m 1/08, 7/26 U.S. Cl. 321-16 7 Claims ABSTRACT OF THE DISCLOSURE Apparatus for hard anodizing using an A.C. power source and incorporating a half wave rectifier and means for cyclically varying the amplitude of rectifier current, including a current limiting resistor and a switch for cyclically connecting the resistor in series with the rectifier output and bypassing the resistor.

This application is a division of my copending application, entitled Anodizing Method and Apparatus, Ser. No. 530,508, filed Feb. 28, 1966, now Patent No. 3,418,222.

This invention relates to electrolytic anodizing of aluminum, including aluminum alloys, and in particular, to a new and improved anodizing apparatus especially suited for producing a hard anodize finish.

A variety of problems are encountered in the production of anodized coatings on aluminum parts, including burning of the parts, excessive time and voltage requirements, and relatively soft finishes. It is an object of the present invention to provide a new and improved apparatus which substantially reduces or eliminates such problems.

It is a particular object of the invention to provide a new and improved apparatus for producing a hard coating while operating for shorter periods of time, at lower electrolyte temperatures and concentrations and at higher current densities. A further object is to provide such an apparatus which will utilize conventional electrolytes either with or without additives.

In the conventional anodizing process, the part to be anodized is placed in a tank of electrolyte and an electric current is passed through the part and electrolyte. The D.C. power supply is connected across the part and tank. The initial voltage typically may be 20 volts D.C. providing a current density at the part in the order of 35 to 50 amperes per square foot. The applied voltage will be periodically increased until the desired coating thickness is obtained. Typically the voltage increase may be 1 voltage per minute except in the critical range where the increase may be 1 volt per two minutes or 1 volt per four minutes. The critical range is the time at which burning of the part is most likely to occur. This is the point in the process when the part is nearly com pletely covered with the initial oxide layer leaving relatively small unoxidized layers having a relatively low surface resistance and hence being susceptibe to a localized high current density which may produce burning of the part. The voltage range in which the critical area occurs for any particular material, electrolyte composition, concentration, tank design and running procedure is readily determined by testing. It is the practice in anodizing processes to make the voltage increases at longer intervals during this portion of the process and also to closely watch the current and voltage indicating meters for sharp variations in current or voltage. Such variations indicate incipient or actual burning and the "ice applied voltage may be reduced to protect the parts. It is a specific object of the invention to provide a new and improved apparatus which substantially eliminates the problems of burning during the critical period and hence substantially eliminates the need for manual supervision of the current and voltage indicators.

It is an object of the invention to provide a new and improved apparatus for operating, an anodizing tank and including a half wave rectifier coupled between an A.C. power source and the tank, and circuit means for cyclically varying the amplitude of the rectifier current in the tank. A particular object is to provide such an apparatus incorporating a current limiting resistance, and switching means for cyclically connecting the resistance in series between the rectifier output and the tank, and bypassing the rectifier output around the resistance.

Other objects, advantages, features and results will more fully appear in the course of the following description. The drawing merely shows and the description merely describes a preferred embodiment of the present invention which is given by way of illustration or example.

In the drawings:

FIG. 1 is an electrical schematic of a preferred form of the apparatus of the invention; and

FIG. 2 is a diagram illustrating the current wave forms produced by the apparatus of FIG. 1.

The present invention utilizes conventional electrolytes and follows the conventional practice of periodically increasing the voltage during the anodizing operation. An additive may be used with the electrolyte if desired. In addition, in the process of the invention, the current is applied in pulses, typically half wave rectified alternating current pulses produced by half wave rectification of the commercial power source. Also, the amplitude of the current pulses is varied at a frequency about ,4 the pulse rate. It has been found that anodizing with pulse current and with periodic variation in amplitude of the current produces a harder coating while permitting operation at lower voltages and for shorter periods of time.

An apparatus suitable for performing the process is illustrated in FIG. 1. A part 10 to be anodized is suspended in a tank of electrolyte 11. The power source may be the commercial volts 60 cycles per second power which is connected at terminals 12, 13. Of course, other power sources can be utilized. The input terminals 12, 13 are connected through an isolating transformer 14 to an autotransformer 15. One terminal of the autotransformer 15 is connected directly to the tank. The moving arm of the autotransformer is connected to a dode unit 16 which functions as a half wave rectifier. The rectifier output is connected to the part 10 through a contact set 17 and an ammeter 18. A variable resistance 19 is connected in parallel with the contact set 17 A voltmeter 20 is connected across the power supply to provide a measure of the voltage at the tank.

Means is provided for cyclically opening and closing the contact set 17. Typically this may comprise a commutator circuit 25 which cyclically energizes and de-energizes a low voltage relay coil 26. Contact set 27 of the low voltage relay controls application of power to coil 28 of a high voltage relay which in turn controls the contact set 17 When the commutator 25 closes the circuit between the 6 volt D.C. supply at terminal 30 and ground, coil 26 is energized, contact set 27 is closed, coil 28 is energized, and contact set 17 is closed and the rectifier output is directly connected to the part being anodized. When the commutator opens the circuit between terminal 30 and ground, contact set 17 is opened, switching the resistance 19 in series with the part and electrolyte. It is recognized that a wide variety of switching devices may be utilized in lieu of the particular circuitry illustrated. The specific embodiment of FIG. 1 provides for commutating at relatively low voltage and current while providing for current control at relatively high voltage and current.

The operation of the apparatus of FIG. 1 is illustrated in FIG. 2. The solid curve 35 illustrates the half wave rectified current pulses applied to the part and electrolyte with the contact set 17 closed. The dashed curve 36 illustrates the current pulses with the contact set 17 open and with the resistance 19 connected in series.

The pulse rate for the current pulses should be in the range of about 45 to about 125 pulses per second. As a matter of economy and convenience, commercial AC. power ordinarily is utilized in the anodizing process and the pulse rate will correspond to the frequency of the power source, which in most instances is 60 cycles per second providing a pulse rate of 60 pulses per second.

The current amplitude is varied at a frequency about the pulse rate and preferably in the range of about 50 to about 150 cycles per minute. The most preferred range utilizing a 60 pulse per second rate is at a frequency of about 70 to about 90 cycles per minute.

In anodizing equipment, the magnitude of the current is a function of the size of the tank, the size of the parts being treated and the number of parts, and hence the actual magnitude of current will vary over a wide range for different installations. Therefore figures on current amplitude are significant only when related to a specific installation. However, the current density at the surface of the part being treated, ordinarily measured in amperes per square foot, does provide for comparison between different installations. At the start of an anodizing run, the current increases from an initial value to a higher operating value. The current remains at this higher operating value for the rest of the run, although it may vary somewhat during the progress of the run. In the process of the invention, the change in current as illustrated in FIG. 2, should be in the range of about 5 to about 17 percent of the operating value and preferably is in the order of 8 to 14 percent, with the most desirable range being about 10 to 12 percent. That is to say, the difference between I and I should be about 11 percent of I The following are specific examples of the process of the invention.

EXAMPLE 1 Test panels of 4 x 4 x inches of 2024-T3 aluminum alloy were anodized using a conventional 10 percent sulfuric acid electrolyte with an additive and operated in the range of 25 to 30 F. The additive was: disodium EDTA, 0.1% of electrolyte by weight; sodium lauryl sulfate, cc. per 40 gallons of electrolyte; and Benax surfactant, 100 cc. per 40 gallons of electrolyte. A half wave rectified 60 cycles per second voltage source was used. The voltage was initially set at 20 volts. The current initially was about 10 amperes and increased to an operating value of about amperes. The current amplitude was cyclically reduced and raised 2 amperes at a frequency of 80 cycles per minute. The voltage was periodically increased as set out in the following chart, producing the coating thicknesses as indicated in the chart.

Coating thickness, inches Raise voltage from- At rate of- 55 .II 1 volt/min 4 EXAMPLE 2 Parts of 6061-16 aluminum alloy were anodized using the same electrolyte, pulse rate, current change frequency and amplitude, and initial starting voltage as in Example 1. The voltage was periodically increased as set out in the following chart to produce the coating thicknesses indicated.

Raise voltage rom- Coating thickness, At rate oi inches 20*30 1 volt/min 30%) 30-55 l volt/min EXAMPLE 3 Parts of 7075-T6 aluminum alloy were anodized in the same manner as set out in Examples 1 and 2, with the voltage being periodically increased as set out in the following chart to produce the thicknesses indicated.

Coating thickness- Raise voltage inches from At rate of- I 1 volt/min 40-55 l volt/min.

Comparative tests between the process of the invention and conventional processes show that the objects of the invention are being achieved. Table I provides a comparison of the time required to produce the specified thickness (in inches) for three different materials.

TABLE I New process Material Conventional 2024-13 002-30 min 002-45-50 min. 003-45 min. 003--80 min. 6061-T6 00220 min 00240 min. 7075-T6 002l520 min 002-35 min.

Conventional (volts) Material The results as set out in Tables I and II were achieved using the new process as set out in Examples 1, 2 and 3 and, for the conventional process, using the same tank and meters with identical parts and with a direct current power supply and a 15 percent sulfuric acid electrolyte at 30 to 35 F. The current density during anodizing in the new process was in the order of to amperes per square foot. The current density in the conventional process was in the order of 35 to 50 amperes per square foot. The coatings produced by the conventional process exhibited a loss of .001 to .0015 inch in a 40,000 cycle Taber test. The coatings produced by the new process exhibited a .0007 to .0008 inch loss in a 40,000 cycle Taber test. No burning problem was encountered during the new process and the process was operated at the high current density. The usual critical control area was encountered in the conventional process, requiring close manual supervision of voltage and current and a limitation on the maximum current density.

The test results show that the process of the invention will produce a harder coating in a shorter period of time and at lower voltages and at lower concentrations of electrolyte than the conventional processes.

Although exemplary embodiments of the invention have been disclosed and discussed, it will be understood that other applications of the invention are possible and that J the embodiments disclosed may be subjected to various changes, modifications and substitutions without necessarily departing from the spirit of the invention.

I claim as my invention:

1. In a current control for operating an aluminum anodizing tank from an A.C. power source, the improvement comprising in combination:

a half wave rectifier coupled between the A.C, power source and the tank;

switching means for cyclically varying the amplitude of the rectifier current in the tank between first and second values of the same order of magnitude, with the frequency of the A.C. power source many times the cyclical rate of said switching means; and

circuit means for connecting said switching means in circuit between the A.C. power source and the tank.

2. An apparatus as defined in claim 1 in which the difference between said second value and said first value is in the range of about 5 to about 17% of said first value.

3. An apparatus as defined in claim 1 in which the frequency of the A.C. source is in the range of about 45 to about 125 cycles per second and the cyclical rate of said switching means is in the range of about 50 to about 150 cycles per minute.

4. An apparatus as defined in claim 1 in which the difference between said second value and said first value is in the range of about 5 to about 17% of said first value, and

in which the frequency of the A.C. source is in the range of about 45 to about 125 cycles per second and the cyclical rate of said switching means is in the range of about 50 to about 150 cycles per minute.

5. An apparatus as defined in claim 1 wherein said switching means includes:

a current limiting resistance; and

6 means for cyclically connecting said resistance in series between the rectifier output and the tank, and bypassing the rectifier output around said resistance. 6. An apparatus as defined in claim 5 wherein said last named means includes a contact set connected in parallel with said resistance, and a commutator for cyclically opening and closing said contact set.

7. An apparatus as defined in claim 1 including means connected to the A.C. power source for varying the voltage of the A.C. input to said rectifier.

References Cited UNITED STATES PATENTS 1,785,389 12/1930 Piersol 204-228 1,806,796 5/1931 Gates 323-96 X 2,262,845 11/1941 Hartley et a1 32396 X 2,930,741 3/1960 Burger et al 204228 X 3,063,929 11/ 1962 Phelan 204228 3,341,445 9/1967 Gerhard 204228 FOREIGN PATENTS 123,234 1/ 1947 Australia.

1,132,985 7/1962 Germany.

JOHN F. COUCH, Primary Examiner W. H. BEHA, JR., Assistant Examiner U.S. Cl. X.R. 

